Removal of high stress zones in electronic assemblies

ABSTRACT

The invention relates to an electronic board (1) comprising: —a printed circuit (2) having a connection face (3) defining a plane (X, Y) comprising at least one transfer area (4), —an electronic component (5) comprising at least one contact terminal (6), each contact terminal (6) being brazed or sintered on an associated transfer area (4) by means of a brazing joint or of a sintering joint (7), the electronic board being characterised in that an orthogonal projection of the contact terminal (6) of the electronic component (5) on the connection face (3) of the printed circuit does not overlap the associated area (4).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/EP2018/083369 filed Dec. 3, 2018, claiming priority based on FrenchPatent Application No. 1762467 filed Dec. 19, 2017, the entire contentsof each of which are herein incorporated by reference in theirentireties.

FIELD OF THE INVENTION

The invention relates to the field of electronic boards, particularly inthe automobile, aeronautics and space fields, and more precisely theattachment of surface-mount components to printed circuits.

TECHNICAL BACKGROUND

In a manner known per se, an electronic board can comprise surface-mountcomponents (SMC), i.e. electronic components soldered directly to thesurface of the printed circuit of an electronic board.

Customarily, SMC are surface mounted either by reflow soldering, or bywave soldering, or by the use of a conductive adhesive, or by sintering.

In the case of reflow soldering, the bare printed circuit is firstscreen printed by covering the conductive layers of the printed circuit(generally made of copper) with a solder paste using a screen (orstencil) so that only the locations intended to receive the terminals ofthe components are covered by the solder paste. The solder pastecomprises, in a manner known per se, a metal alloy in suspension in asoldering flux. Then the terminals of the (SMC) components are laid onthe solder paste before undergoing a thermal reflow treatment, duringwhich the heat melts the alloy and evaporates the soldering flux so asto form solder joints from the metal alloy present in the solder paste.

In the case of assembly by silver sintering, the bare printed circuit isalso screen printed by covering the conductive layers of the printedcircuit with a paste by means of a screen (or stencil) so that only thelocations intended to receive the terminals of the components arecovered by the paste. The paste comprises, in a manner known per se,particles of silver and organic elements serving as binders, dispersersand diluents. Then the terminals of the (SMC) components are positionedon the paste before undergoing a thermal sintering treatment, duringwhich the heat welds the grains together without leading them tomelting, thus producing the joint.

The joints thus obtained (whether by soldering or by sintering) arehowever subjected to significant stresses. In particular, the portion ofthe joints which is positioned between the component and the solder padis very thin, as can be seen in FIG. 1. The joints are thereforesubjected to very strong stresses, particularly in shear, and thereforerisk cracking rapidly when the electronic board undergoes extremeenvironments of temperature and/or vibration. This therefore reducessharply the lifetime of the electronic board.

Moreover, in use, the electronic boards can be subjected to extremetemperature environments creating relative displacements (in the planeof the surface) between the SMCs and the printed circuit. These relativedisplacements are generally due to a differential expansion between theSMCs and the printed circuit, resulting from a variation of temperature,the SMCs and the printed circuit generally having different thermalexpansion coefficients in the plane. Yet these relative displacementshave the effect of generating stresses which damage the solder joints,thus limiting their lifetime and their reliability.

In order to reduce these stresses, it has been proposed to increase thevertical height (usually designated by the term “standoff”) between theupper face of the solder pad (which can be of copper, of copper coveredwith nickel-gold or of any other suitable material) and the lower pointof the conductive terminals of the SMCs once soldered, in order toincrease the height of the alloy at the interface between the SMCs andthe solder pads. The solder joints thus obtained are then more flexibleand therefore more robust.

However, the aeronautics and space industry keeps constantly searchingfor improvements in electronic boards, and therefore is still searchingfor inventions to improve the reliability and the lifetime of SMC solderjoints.

SUMMARY OF THE INVENTION

One objective of the invention is therefore to propose an electronicboard comprising a printed circuit and surface-mount components havingimproves lifetime, reliability and toughness, which is easy to produceand has moderate cost regardless of the density of components installedon the printed circuit and/or the type of component, without howeverimpacting the assembly efficiency of the electronic board and withoutmodifying the assembly method usually used.

To this end, the invention proposes an electronic board comprising:

a printed circuit having a connection face defining a plane includingtwo solder or sintering pads, said pads being separated by a firstdistance when the electronic board is at ambient temperature, theprinted circuit having a first thermal expansion coefficient in theplane,

an electronic component including at least one terminal, each terminalbeing soldered or sintered on an associated pad by means of a solder orsintered joint, the electronic component comprising two terminals, saidterminals being separated by a second distance when the electronic boardis at ambient temperature, the electronic component having a secondthermal expansion coefficient in the plane, the solder joint having athird thermal expansion coefficient in the plane,

the electronic board is characterized in that the first distance issubstantially equal to the quotient of the difference between the firstthermal expansion coefficient and the third thermal expansioncoefficient and of the difference between the second thermal expansioncoefficient and the third thermal expansion coefficient, all multipliedby the second distance:

$L_{PCB} = {L_{CMS} \times \frac{{CTE_{SMC}} - {CTE_{J}}}{{CTE_{PCB}} - {CTE_{J}}}}$so that an orthogonal projection of the terminal of the electroniccomponent on the connection face of the printed circuit does not overlapthe associated pad.

Certain preferred but non-limiting features of the electronic boarddescribed above are the following, taken individually or in combination:

the pad extends under the electronic component or on the side of theelectronic component.

the electronic board also comprises an insulating layer covering atleast partially the connection face, at least one cavity formed in theinsulating layer and exposing at least partially the pad, the electroniccomponent being in contact or at a distance from the insulating layerand the terminal of the electronic component being joined to thecorresponding pad through the cavity.

the printed circuit comprises at least two pads and the electroniccomponent comprises at least two terminals associated with said pads,the printed circuit also comprising at least four conductive layersseparated two by two by insulating layers, including a first skinconductive layer joined to the insulating layer, a first and a secondinternal conductive layers comprising the two pads, and at least twocavities formed in the first skin conductive layer and in the insulatinglayer.

the wall of the cavity is substantially perpendicular to the pad.

the electronic board also comprises an adhesive point under theelectronic component, between the pads, in order to retain theelectronic component in position relative to the pads during solderingof the electronic component to the printed circuit.

the electronic board also comprises, for each terminal, a sacrificialpad, said sacrificial pad extending between the pad and a portion of theprinted circuit which extends facing the terminal.

the sacrificial pad has a dimension, in a direction corresponding to adirection of greater expansion of the electronic component, equal to atleast 200 microns.

the electronic board also comprises a connection strip configured toconnect electrically the sacrificial pad and the associated pad.

the sacrificial pad, the pad and the connection strip form an H, a U oran O.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, aims and advantages of the present invention will appearmore clearly upon reading the detailed description which follows, andwith reference to the appended drawings, given by way of non-limitingexamples and in which:

FIG. 1 illustrates schematically an exemplary embodiment of anelectronic board conforming to the prior art,

FIG. 2a is a schematic view of a first exemplary embodiment of anelectronic board conforming to the invention comprising a surface-mountcomponent, in the case where the first distance is less than the seconddistance. The electronic board is at ambient temperature.

FIG. 2b illustrates the electronic board when it is subjected to aextreme temperature.

FIG. 3 is a schematic view of a second exemplary embodiment of anelectronic board conforming to the invention, comprising a surface-mountcomponent, in the case where the first distance is greater than thesecond distance.

FIG. 4 is a schematic view of a third exemplary embodiment of anelectronic board conforming to the invention, in which the cavities areformed in an insulating layer covering all or a portion of theconnection face of the insulating board and the first distance is lessthan the second distance.

FIG. 5 illustrates a fourth embodiment of an electronic board conformingto the invention, in which an adhesive point is used to retain theelectronic component during its soldering to the printed circuit.

FIGS. 6a and 6b illustrate two exemplary embodiments of a fifthembodiment conforming to the invention, in which the electroniccomponent is also joined to sacrificial solder pads.

FIGS. 7a and 7b are schematic top-views illustrating a variantembodiment of the fifth embodiment, in which the sacrificial solder padsare connected to the solder pads by means of one or of two connectionstrips.

DETAILED DESCRIPTION OF ONE EMBODIMENT

An electronic board 1 comprises a printed circuit 2 including conductivelayers separated by insulating layers on which are mounted surface-mountelectronic components 5 (hereafter SMCs 5).

Generally, a printed circuit 2 can be of the one-layer type (also calledsingle layer) and comprise only a single conductive layer over aninsulating layer, double layer (also called double face) and comprise aconductive layer on either side of an insulating layer or multi-layerand comprise at least four conductive layers (and therefore threeinsulating layers).

The printed circuit 2 has a connection face 3 defining a plane (X, Y)and intended to receive at least one SMC 5. To this end, the connectionface 3 includes at least one pad 4. What is meant here by pad 4 is apad, of copper for example, on which the SMC 5 is soldered or sintered.

The SMCs 5, for their part, comprise at least one terminal 6, eachterminal being joined to an associated pad 4 of the connection face 3 ofthe printed circuit 2 by means of a solder joint 7.

Hereafter, the invention will be described in the case where the printedcircuit 2 of the electronic board 1 comprises two solder pads 4, twosolder joints 7 and one SMC 5 comprising two terminals 6. This, however,is not limiting; the printed circuit can comprise a greater number ofsolder pads 4, or can be obtained by sintering and then includesintering pads 4 and as many associated sintering joints 7, theelectronic board 1 being able to comprise a greater number of SMCs 5,and the SMCs 5 a different number of terminal (at least one terminal 6).The invention then applies mutatis mutandis to each assembly formed from(solder or sintering) pads 4 and the associated SMC 5.

The invention also applies mutatis mutandis to sintering or soldering achip to a substrate in a surface-mount component, because said chipsthemselves form a surface-mount component on a printed circuit.

In order to improve the lifetime of the electronic board 1 and to reducethe stresses in the solder joints 7, the solder pad(s) 4 are positionedon the connection face 3 of the printed circuit 10 so that an orthogonalprojection of the terminals 6 of the electronic component 5 on theconnection face 3 of the printed circuit does not overlap the associatedsolder pad(s) 4. In other words, the solder pads 4 and the terminals 6do not overlap, so that the solder pads 4 are located under the SMC 5 oron either side of the SMC 5.

This therefore allows eliminating the zone of the solder joint 7 whichis positioned between a given terminal 6 of the SMC 5 and the associatedsolder pad 4. As we have seen above, this very thin zone (on the orderof ten microns) corresponded in fact to the location of the solder joint7 where the shear stresses (and therefore the risks of cracking) aregreatest in an SMC mounted to the printed circuit in accordance with theprior art (see FIG. 1). Yet by eliminating this zone, the shear stressesin the solder joint 7 are homogenized and the risks of cracking arelimited, which further improves the lifetime of the SMC assembly.

In the case where the printed circuit 10 and the SMC 5 have differentthermal expansion coefficients in the plane (X, Y), the distance betweenthe solder pads 4 can also be adjusted depending on the distance betweenthe terminals 6 of the SMC and the value of these thermal expansioncoefficients so as to optimize their relative displacement during theexposure of the electronic board 1 to extreme temperature conditions andto relieve the solder joints 7.

In fact, the solder pads 4 of the printed circuit are separated by afirst distance L_(PCB). This first distance L_(PCB) is measured atambient temperature T₁ between the respective centers 4 a of the solderpads 4.

The terminals 6, for their part, are separated by a second distanceL_(CMS). This second distance L_(CMS) is measured at ambient temperatureT₁ between the respective ends 6 a of the terminals 6. In the case wherethe SMC 5 comprises more than two terminals 6, the second distanceL_(CMS) corresponds to the distance between the ends 6 a of the mostdistant terminals 6, typically diagonally. Moreover, for the SMCs 5whose terminals comprise tabs, the distance L_(CMS) extends from thefree end of the tabs which faces the solder pads 4 and not from theirend joined to the SMC.

Finally, the solder joints 7 have a cumulative thickness L_(J) measuredat ambient temperature T₁ between their free edges. In the case wherethe SMC 5 comprises more than two terminals 6, the thickness L_(J)corresponds to the cumulative thickness of the solder joints 7 betweenthe ends 6 a of the most distant terminals 6, typically diagonally onthe SMC 5.

The expansion ΔL_(PCB) of the printed circuit 2, the expansion ΔL_(CMS)of the SMC 5 and the expansions ΔL_(J) as functions of temperature(these expansions ΔL_(PCB), ΔL_(CMS) and ΔL_(J) being able to bepositive and/or negative) can be evaluated thanks to the followingrelations:ΔL _(CMS) =L _(CMS) ×ΔT×CTE _(SMC)ΔL _(PCB) =L _(PCB) ×ΔT×CTE _(PCB)ΔL _(J) =L _(J) ×ΔT×CTE _(J)where L_(PCB) is the first distance

L_(CMS) is the second distance

L_(J) is the cumulative thickness of the solder joints 7

ΔT=T₂−T₁

T₁ is the ambient temperature at which the first and second distancesL_(PCB), L_(CMS) are determined. What is meant here by ambienttemperature T₁ is a temperature on the order of twenty degrees.

T₂ is a different temperature from T₁, for example a utilization,storage or operating temperature of the electronic board 1,

ΔL_(CMS) is the expansion of the SMC 5 between the ambient temperatureT₁ and the temperature T₂,

ΔL_(PCB) is the expansion of the printed circuit 2 between the ambienttemperature T₁ and the temperature T₂,

ΔL_(J) is the expansion of the solder joints 7 between the ambienttemperature T₁ and the temperature T₂.

The difference ΔL_(assembly) between the expansion of the SMC 5 and theexpansion of the printed circuit 2, between the ambient temperature T₁and the temperature T₂, can therefore be obtained thanks to thefollowing formula (1):ΔL _(assembly)=(L _(PCB) ×CTE _(PCB) −L _(CMS) ×CTE _(SMC) −L _(J) ×CTE_(J))×ΔT

Moreover, the distance L_(PCB) is equal to the sum of the distanceL_(CMS) and the cumulative thickness L_(J) of the solder joints 7. Thefollowing formula (2) is therefore obtained:ΔL _(assembly)=(L _(PCB)(CTE _(PCB) −CTE _(J))−L _(CMS)(CTE _(SMC) +CTE_(J))×ΔT

It is thus possible to simply and effectively increase the lifetime andthe toughness of an electronic board 1 by adjusting the first distanceL_(PCB) depending on the second distance L_(CMS) and the thermalexpansion coefficients CTE_(PCB), CTE_(J) and CTE_(SMC), thanks to theformula (1) reproduced above. Regardless of the operating temperature,the difference in expansion ΔL_(assembly) is therefore optimized so asto be as small as possible, thus strongly limiting the stresses withinthe solder joints 7.

The second distance L_(CMS) being fixed and intrinsic to the SMC 5, thefirst distance L_(PCB) is therefore adjusted by engraving the solderpads 4 on the connection face 3 so that the first distance L_(PCB)allows compensating the difference in the thermal expansion coefficientbetween the printed circuit 2, the solder joints 7 and the SMC 5.

Ideally, a zero difference in expansion ΔL_(assembly) is sought. Inorder to reduce the stresses formed within the solder joints 7 duringvariations in temperature, the first distance L_(PCB) (between thesolder pads 4) is therefore selected so as to be equal to the quotientof, on the one hand, the difference between the thermal expansioncoefficient of the SMC (CTE_(SMC)) and the thermal expansion coefficientof the solder joints and, on the other hand, the difference between thethermal expansion coefficient of the printed circuit board (CTE_(PCB))and the thermal expansion coefficient of the solder joints, allmultiplied by the second distance L_(CMS):

$\begin{matrix}{L_{PCB} = {L_{CMS} \times \frac{{CTE_{SMC}} - {CTE_{J}}}{{CTE_{PCB}} - {CTE_{J}}}}} & (3)\end{matrix}$

It will be noted that the invention finds application particularly whenthe difference between the first thermal expansion coefficient(CTE_(PCB)) and the second thermal expansion coefficient (CTE_(SMC)) isgreater than or equal to 2. Outside this range, the differentialexpansion of the SMC 5 and of the printed circuit is in fact too low forthe relative displacement of the SMC 5 and of the printed circuit to benoticeable.

Thus, when the thermal expansion coefficient CTE_(SMC) of the SMC 5 isless than the thermal expansion coefficient CTE_(PCB) of the printedcircuit 2, which itself is less than the thermal expansion coefficientCTE_(joint) of the joint 7 (case of the great majority of SMCs 5), it isdiscovered that the first distance L_(PCB) is strictly greater than thesecond distance L_(CMS) (FIG. 3) so that the solder pads 4 are locatedon either side outside the terminals 6 of the SMC 5. In this manner,when the electronic board 1 undergoes a temperature T₂ greater than theambient temperature T₁, the SMC 5 and the printed circuit 2 dilate.However, the thermal expansion coefficient CTE_(PCB) of the materialconstituting the printed circuit 2 being greater than that of the SMC 5,the printed circuit dilates more than the SMC 5. The distance L_(PCB)being strictly greater than the second distance L_(CMS), the result isthat at this temperature T₂, the solder pads 4 and the terminals 6 ofthe SMC 5 are essentially at the same relative distance as at ambienttemperature T₁, which relieve the stresses in the solder joints 7relative to the case where the solder pads 4 and the terminals 6 of theSMC 5 are facing one another at ambient temperature T₁ (FIG. 1) and therelative distance between them varies more substantially.

Thus raising the temperature does not modify the relative displacementbetween the terminals 6 of the SMC 5 and the solder pads 4 of theprinted circuit 2 and reduces its negative effects by compensating thedifference in expansion of the SMC 5, of the joints 7 and of the printedcircuit 2.

In practice, the lifetime and the toughness of the electronic board 1are already considerably improved when the first distance L_(PCB) isequal, within 50%, to the multiplication of the quotient of thedifferences in thermal expansion coefficients

$\frac{{CTE_{SMC}} - {CTE_{J}}}{{CTE_{PCB}} - {CTE_{J}}}$by the second distance L_(CMS) (formula (3) described above), as long asthe orthogonal projection of the terminals 6 does not overlap theassociated solder pad 4. This tolerance of 50% is justified by thefollowing observations:

The terminal of the SMC 5 (respectively, the printed circuit 2) has avariable expansion ΔL_(CMS) (respectively, ΔL_(PCB)) between its twoends (respectively between the edges of the solder pad 4). It istherefore not possible to obtain

$L_{PCB} = {L_{CMS} \times \frac{{CTE_{SMC}} - {CTE_{J}}}{{CTE_{PCB}} - {CTE_{J}}}}$everywhere in the solder joint.

It is not always possible to bring the solder pads 4 sufficiently closetogether to satisfy the formula (3). For example, if the thermalexpansion coefficient of the SMC 5 is twice higher than that of theprinted circuit 2, it is not possible to bring the pads sufficientlyclose together to satisfy the formula (3) and produce the solder jointduring manufacture of the electronic board 1.

The thermal expansion coefficients of the printed circuit, of the solderjoints 7 and of the SMC are generally not known with accuracy and have adispersion.

The thermal expansion coefficients of the printed circuit, of the solderjoints 7 and of the SMC vary depending on the temperature.

For example, for single-face printed circuits 2, the thermal expansioncoefficient CTE_(PCB) of the printed circuit 2 is generally comprisedbetween 14 ppm/° C. and 17 ppm/° C. In the example below, the printedcircuit 2 has a thermal expansion coefficient CTE_(PCB) equal to 14ppm/° C. The solder joint 7 is produced with a leadless alloy SAC305,the thermal expansion coefficient CTE_(J) of which is 23.5 ppm/° C. TheSMC 5 is a 2512 resistor with a length of 6.3 mm so that the seconddistance L_(CMS), which corresponds to the distance between the ends 6 aof the terminals 6 of the resistor, is equal to 6.3 mm at ambienttemperature. Moreover, the thermal expansion coefficient CTE_(SMC) ofthe resistor is equal to 7. It is therefore attempted to separate thesolder pads 4 so that the second distance L_(CMS), which corresponds tothe distance between the centers of the solder pads 4, approaches thetheoretical value that is obtained thanks to formula (3), orapproximately 10.9 mm:

${L_{PCB} = {{{6.3} \times \frac{7 - {2{3.5}}}{{14} - {2{3.5}}}} = {1{0.9}}}}\mspace{14mu}{mm}$

In one embodiment, the electronic board 1 can also comprise aninsulating layer 8 covering at least partially the connection face 3 andtwo cavities 9, formed in the insulating layer and exposing at leastpartially the solder pads 4. In this case, the SMC 5 is in contact withor at a distance from the insulating layer 8 and its terminals 6 arejoined to the corresponding soldering pads 4 through the first andsecond cavities 9.

An embodiment of this type is described more particularly in Frenchpatent application no. 17 56700, filed on 13 Jul. 2017 in the name ofthe Applicant. The production of cavities 9 of this type allows simplyincreasing at low cost the standoff of the SMC 5 without, however,impacting the assembly efficiency of the SMC 5.

In one variant of this embodiment (FIG. 4), the electronic board 1 ismulti-layer and comprises at least four conductive layers separated, twoby two, by insulating layers, including:

a first skin conductive layer joined to the insulating layer,

a first and a second internal conductive layers comprising two solderpads 4.

The two cavities 9 are then formed in the first skin conductive layerand in the insulating layer.

French patent application no. 17 56679 in particular can be referred to,filed on 13 Jul. 2017 in the Applicant's name. An electronic board 1 ofthis type can thus comprise various SMCs 5, whether fine-pitchcomponents, large size components, with seagull tabs, etc., simply andat moderate cost without, however, impacting the assembly efficiency ofthe SMCs 5.

Regardless of the variant embodiment, the wall 10 of the cavities 9 canbe inclined relative to the connection face 3. As a variant, it candiverge. According to yet another variant the wall 10 of the cavities 9is perpendicular to the connection face 3. In this last variantembodiment, the exposed surface of the connection face 3 is thereforelarger than the surface of the solder pads 4. If applicable, the cavity9 can be larger in the principal direction of expansion of the SMC 5than the solder pad 4. To this end, the insulating layer can bepre-drilled before its assembly with the printed circuit 2. As avariant, the solder pad 4 can be produced so as to be larger thannecessary in this direction, then the cavity 9 can be produced bysurface photolithography as described in document FR 17 56700 and thesuperfluous zone of the solder pad 4 can be withdrawn.

During automatic joining, particularly by reflow, of an SMC 5 on aprinted circuit 2, it may happen that the SMC 5 is not centered andmoves more toward one of the terminals 6 due to wetting forces which aregenerally not balanced (the SMC 5 generally not being placed exactly inthe middle of the two pads of solder paste).

In the event of displacement of the SMC 5, the desired effect is thennot obtained: typically, this displacement can re-create a zone ofstrong stresses in the solder joint 7 vertically between a giventerminal 6 of the SMC 5 and the associated solder pad 4, which isconducive to the initiation of cracks and therefore reduces the lifetimeof the SMC. Worse still, the SMC 5 can separate from one of theterminals 6 during soldering.

Two embodiments are proposed to facilitate the automatic manufacture ofelectronic boards 2. This problem does not occur, in fact, during manualjoining of the SMC 5.

In a first embodiment (FIG. 5), an adhesive point 11 is placed betweenthe SMC 5 and the solder pads 4 of the printed circuit 2 prior to thesoldering step. In fact, one adhesive point 11 of this type issufficient to retain the SMC 5 in position and oppose the wettingforces.

Typically, the adhesive 11 is selected so as to have a polymerizationtemperature lower than the heat treatment temperature used duringsoldering of the SMC 5. For example, when the SMC 5 is soldered byreflow of a solder paste, the adhesive is selected so as to have apolymerization temperature on the order of 100° C. to 150° C. (recallthat soldering by reflow comprises, in a manner known per se, thesuccessive steps of temperature rise, preheating, reflow and cooling,the temperature of the reflow step being generally higher than 180° C.).In this manner, the adhesive polymerizes during the preheating phase andis capable of retaining the SMC 5 in position during the reflow step.

The adhesive can for example be an epoxy adhesive.

In a second embodiment (FIGS. 6a, 6b, 7a and 7b ), a sacrificial solderpad 12 is added, for each terminal 6 of the SMC 5, between the solderpad 4 and the portion of the printed circuit 2 which extends facing theassociated terminal 6. The sacrificial pad 12 can be produced in thesame material as the associated solder pad 4. Thus, the sacrificial pads12 can be placed on the outside of the solder pads 4 when the solderpads 4 are located under the SMC 5, or between the solder pads 4 whenthe solder pads 4 are located on the outside of the SMC 5.

Preferably, the sacrificial solder pads 12 are very thin (for example onthe order of two hundred microns) so as to retain the advantage linkedto the offset of the solder pads 4 relative to the center of theterminals 6. When the electronic board 2 undergoes an extreme thermalenvironment, the shear stresses will then be concentrated in the portionof the solder joint 7 which is joined to the sacrificial pads 12, andwill generally crack them. However, these sacrificial pads 12 beingdistinct from the solder pads 4, the cracking will not affect theportion of the solder joint 7 which is joined to the solder pads 4. Wewill therefore find an electronic board 1 in which the orthogonalprojection of the terminals 6 does not overlap the solder pads 4. Inparallel, the sacrificial pads 12 are capable of retaining the SMC 5 inposition and avoiding having it offset itself during its soldering tothe printed circuit board 2.

The Applicant observed that, in very rare cases, the SMC 5 could moveanyway and then be joined at only one of its terminals 6 to one of thesacrificial pads 12. In order to further reduce reject rates, for eachterminal 6, the sacrificial pad 12 and the associated solder pad 4 canbe produced by a thin strip 13 produced from the same material as thesolder pad 4. Untimely displacements of the SMC 5 and of the solderjoint 5 during soldering of the SMC 5 to the printed circuit are thenrarer. In the event of an untimely displacement of the SMC 5 and of thesolder joint 5 during soldering of the SMC 5 to the printed circuit sothat the solder joint 7 is no longer in contact other than with thesacrificial pad 12, the electrical connection with the solder pad 4remains assured thanks to the thin strip 13. The thin strip 13 can becentered overall between the sacrificial pad 12 and the solder pad 4(thus forming an H—FIG. 6a ), or extend along one of their edges (thusforming a U). As a variant, two strips 13 can extend between thesacrificial pad 12 and the solder pad 4, for example at each of theedges of the solder pad 4 (thus forming an O—FIG. 6b ).

It will be noted that the shape and the dimensions of the sacrificialsolder pad 12 are not limiting. In particular, the sacrificial solderpad 12 can be longer or shorter than the solder pad 7, wider ornarrower, without this being limiting.

The invention claimed is:
 1. An electronic board comprising: a printedcircuit having a connection face defining a plane including two, the twopads being separated by a first distance when the electronic board is atambient temperature, and an electronic component including two terminal,each terminal being soldered or sintered on an associated pad of the twopads by means of a solder or sintering joint, the two terminals beingseparated by a second distance when the electronic board is at ambienttemperature, wherein the printed circuit having a first thermalexpansion coefficient (CTE_(PCB)) in the plane, the electronic componenthas a second thermal expansion coefficient (CTE_(SMC)) in the plane andthe solder joint having a third thermal expansion coefficient (CTE_(J))in the plane, and wherein the first distance (L_(PCB)) is substantiallyequal to a quotient of a difference between the first thermal expansioncoefficient (CTE_(SMC)) and the third thermal expansion coefficient(CTE_(J)) and of a difference between the second thermal expansioncoefficient (CTE_(PCB)) and the third thermal expansion coefficient(CTE_(J)), all multiplied by the second distance (L_(CMS)):${L_{PCB} = {L_{CMS} \times \frac{{CTE_{SMC}} - {CTE_{J}}}{{CTE_{PCB}} - {CTE_{J}}}}},$so that orthogonal projections of the two terminals of the electroniccomponent on the connection face of the printed circuit do not overlapthe associated pads of the two pads.
 2. The electronic board accordingto claim 1, wherein the pad extends under the electronic component or ona side of the electronic component.
 3. An electronic board according toclaim 1, further comprising: an insulating layer covering at leastpartially the connection face, and a cavity formed in the insulatinglayer and exposing at least partially one of the two pads, theelectronic component being in contact or at a distance from theinsulating layer and the terminal of the electronic component beingjoined to the at one of the two pads through the cavity.
 4. Theelectronic board according to claim 3, wherein the printed circuitfurther comprises: a first skin conductive layer joined to theinsulating layer, a first and a second internal conductive layerscomprising the two pads, and an additional cavity formed in the firstskin conductive layer and in the insulating layer.
 5. The electronicboard according to claim 3, wherein a wall of the cavity issubstantially perpendicular to the one of the two pads.
 6. An electronicboard according to claim 1, further comprising an adhesive point underthe electronic component, between the two pads, in order to retain theelectronic component in position relative to the two pads duringsoldering of the electronic component to the printed circuit.
 7. Theelectronic board according to claim 1, further comprising, for each ofthe two terminal, a sacrificial pad extending between a correspondingpad of the two pads and a portion of the printed circuit which faces theterminal.
 8. The electronic board according to claim 7, wherein thesacrificial pads have a dimension, in a direction corresponding to adirection of greater expansion of the electronic component, equal to atleast 200 microns.
 9. The electronic board according to claim 7, furthercomprising a connection strip configured to electrically connect thesacrificial pads and the corresponding pad of the two pads.
 10. Theelectronic board according to claim 9, wherein the connection strip iscentered between the sacrificial pad and the corresponding pad of thetwo pads, or extends between two opposite ends of the sacrificial padand the corresponding pad of the two pads.
 11. The electronic boardaccording to claim 9, further comprising an additional connection strip,wherein the connection strip and the additional connection strip extendfrom opposite ends of the sacrificial pad and the corresponding pad ofthe two pads.